Film treating method

ABSTRACT

A method is disclosed for the surface treatment of a plastic body by exposure to a high intensity voltage accompanied by corona discharge wherein said voltage is a sequence of alternating-directional, sonic frequency pulses of electrical voltage.

United States Patent 1 Rosenthal et al.

11 3,736,492 [451 May 29, 1973 [54] FILM TREATING METHOD [75] Inventors:Louis A. Rosenthal, Highland Park; Donald A. Davis, Somerville, both ofNJ.

[73] Assignee: Union Carbide Corporation, New

York, NY.

[22] Filed: Jan. 14, 1971 [21] Appl. No.: 106,376

Related US. Application Data [63] Continuation-in-part of Ser. No.862,412, Sept. 30,

1969, abandoned.

[52] US. Cl. ..321/45 R, 250/495 GC, 250/49.5 TC, 204/312 [51] Int. Cl...H02m 7/48 [58] Field of Search ..250/49.5; 321/45, 321/45 C; 204/312[56] References Cited UNITED STATES PATENTS 3,514,393 5/1970 Eisby..204/3l2 3,303,406 2/l967 Bedford.... .....32l/45 R 3,496,092 2/1970Fraser ..32l/45 X 3,294,971 12/1966 Von Der Heide .....250/49.52,969,463 l/l96l McDonald ..250/49.5

3,047,789 7/l962 Lowry ..32 1 I45 3,263,l53 7/l966 Lawn ..32l/45 C3,391,314 7/1968 Carter ..3 17/262 A OTHER PUBLICATIONS PrimaryExaminer-William M. Shoop, Jr. Attorney-Paul A. Rose, Gerald R. OBrien,Jr. and Aldo John Cozzi [57] ABSTRACT A method is disclosed for thesurface treatment of a plastic body by exposure to a high intensityvoltage accompanied by corona discharge wherein said voltage is asequence of altemating-directional, sonic frequency pulses of electricalvoltage.

2 Claims, 3 Drawing Figures Patented May 29, 1973 3 Sheets-Sheet 1 INENTORS Louis A. 26 Donald A. 89$?" AT ORNEY Patented May 29, 19733,736,492

3 Sheets-Sheet 2 FIG. 2.

INVEN'IORS Louis A. Rosenthol BY Donald A. Davis Patented May 29, 1973CORONA LOAD IN WATTS (AT GENERATOR) AT |20V 0.0.

3 Sheets-Sheet 3 Louis A. Rosenthol Donald A. Davis FILM TREATING METHODThis application is a continuation-in-part of application Ser. No.862,412, filed Sept. 30, 1969 and now abandoned.

BACKGROUND OF THE INVENTION Exposing the surface of a polymer body, suchas polyethylene film, to a high voltage gaseous discharge having coronacharacteristics is known to improve the affinity of the surface foradhesives, inks and other polar substrates. The treatment zone of atypical system comprises a relatively large ground electrode separatedfrom one or more relatively sharp high voltage elec' trodes by two andpreferably three dielectrics. The essential dielectrics are anio'nizable gaseous dielectric, normally air, and the polymeric body tobe treated. Normally, the ground electrode is covered with a bufferdielectric, such as rubber or a polyester film, which acts to precludean arc from bridging the gap at weak points in the polymer body. Thehigh voltage electrode, which may consist of one or more treater bars inseries or in parallel, runs the length of the ground electrode and is incircuit with a high voltage generator.

Most commercial treating systems employ alternating current supplied atfrequencies up to 500 kHz or more. Gap voltages up to kv or more areemployed to effectively treat a polymer film which is continuouslypassed through the gap at speeds up to 500 feet per minute or more. Inpractice, an energy density-to-film surface of the order of about 1watt-minute per square foot of film surface or more is sought to achievegood surface adhesion characteristics.

While every component of a film treating system has come underinvestigation from time to time, the waveform of the high voltageemployed in the treating system has generally been neglected. Thespark-gap generators and motor alternators now in use are inefficientand suffer from many inherent deficiencies.

In addition to interfering with radio reception due to the presence ofradio frequencies in the spark-gap generator output wave, that generatorhas a short duty cycle. The range of output power for a given generatoris severely limited since the gap breakdown voltage sets the minimumvoltage.

The motor alternator, on the other hand, is cumbersome in size andsubject to frequent mechanical failure. Further, its output issinusoidal which is far from the ideal waveform.

In a typical high voltage film treating system, an alternating currentline voltage is fed to a high voltage generator and the generatoralternating current output is fed through an output transformer to thetreating circuit load.

The load should be viewed as a lossy capacitor wherein the electrodes,in their area and spacing, define the capacitance and the dielectric isa composite made up of an air gap, the film and the buffer dielectricall in series. Asthe corona voltage threshold level is reached, thelosses of this system vary in a nonlinear manner. It is the losscomponent which is effective in treatment and the recognition of thecapacitive reactive behavior of the load is important.

The concept of variable frequency has been only recently recognized asthe all important parameter for load adjustment and optimization in filmtreating operations. Looking at the corona treating region as a lossycapacitor system, the power would be proportional to frequency just as,for a given input voltage, the current entering a capacitor is linearwith frequency. This concept is disclosed and claimed in our copendingapplication, filed of even date herewith, and entitled Film TreatingProcess.

SUMMARY OF THE INVENTION The present invention relates to the highvoltage surface treatment of a plastic body with an alternating voltageof high intensity accompanied by corona discharge, wherein the treatmentzone constitutes a ca pacitive load, comprising exposing said surface toan alternating-directional pulse waveform electrical voltage in thesonic frequency range and accompanying corona discharge.

It has been found that a broad range of sonic frequency (20-20,000 Hz)treating voltages may be employed, where frequency is varied to effectsurface treatment under optimum load conditions. Accordingly, a treatingsystem providing a broad frequency variation of treating voltage over arange of 20 to 5,000 Hz is desired.

The pulse waveform is desired since corona current flows only during thetime of voltage change according to i C(dv/dt). For example, a pulsewith its rapid positive and negative change will result in coronacurrent. The square wave during the flat top region results in no coronacurrent and merely applies an electrical stress on all components. Thenature of a corona load is such that charges residing on the dielectricsurfaces inhibit further corona discharge during the constant voltageregion. Thus, the square wave is of no value and only the swing frompositive to negative and negative to positive extremes is useful ingenerating corona. Similar arguments can be applied to the sine wavewaveform. It has been discovered that the transient aspect is theessential part of any waveform and the most desirable from the point ofview of efficacy and utilization.

In the drawings:

FIG. 1 is a schematic view of apparatus circuitry capable of use in thepractice of the process of the invention;

FIG. 2 (a) and (b) are schematic representations of the treating circuitvoltage and load current waveforms, respectively, for apparatus of thetype shown in FIG. 1;

FIG. 3 is a graphical representation of the relationship betweentreating load power and frequency employed for varying electrode lengthsin the process of the present invention.

An improved film treating system is shown schematically in FIG. 1 of thedrawings. As there shown, a suitable variable direct current source isprovided comprising a variable autotransformer 10 having an alternatingcurrent supply, the output of which is rectified by a full waverectifier 12 and filtered by capacitor 14 connected across the outputterminals of rectifier 12. The dc voltage output E which is a directfunction of the applied auto transformer voltage is fed to the highvoltage pulse output circuit 15. Polyphase rectifiers and the like canalso be used to provide adjustable dc voltages. It should be noted,however, that the employment of means for varying input voltage andconsequent selected output voltage to a desired constant levelconstitutes merely an apparatus convenience but does not constitute apoint of criticality or novelty in the present invention.

The high voltage pulse output circuit comprises a high voltagetransformer 16 having a high voltage secondary winding and a low voltageprimary center tapped at 18 where voltage E is applied. At least twopower thyristors 20 and 22 are coupled at their cathodes andrespectively connected at their anodes to the end taps 24 and 26 of theprimary of the transformer 16. As described in the article Thyristors:Semiconductors for Power Control" by V.W. Wigotsky in Design News, Vol.22, No. 18, page 26, which is incorporated by reference, thyristors aresuper switches for electrical power as is their function in the solidstate high voltage generator of this invention. The preferred powerthyristors are silicon controlled rectifiers but any solid state deviceor combination of devices which function equivalent to a thyristor orswitch can be used. Ordinarily, a thyristor, particularly asilicon-controlled rectifier in a high conductive state, continues toconduct after the gate signal is removed until the anode current isinterrupted or diverted for a time sufficient to permit the rectifier toregain its forward blocking condition.

At least one capacitor 28 is connected across the end taps 2a and 26 ofthe primary of the transformer and consequently between thyristors 20and 22.

The high voltage transformer 16 is an important integral part of thehigh voltage pulse generator circuit. It is center-tapped with endreturn taps in the primary while the secondary is a high potentialwinding. Its core must not saturate at operating frequencies andvoltages.

At least one pair of diodes 34 and 36 are, respectively, connected attheir cathodes to the end taps 24 and 26 of the primary of thetransformer 16. The anodes of diodes 34 and 36 are commonly connected tothe cathodes of thyristors 20 and 22. Inductor 38 is positioned betweenfiltering capacitor 14 and pulse output circuit 15. The diodes 34 and 36act as anti-parallel or reverse conduction diodes to allow for reversedcurrent flow.

The rate at which power thyristors undergo gating is controlled by atiming circuit 40 which is typically a multi-vibrator, preferably afree-running, astable, solid state oscillator or a unijunction, astableoscillator which generates trigger pulses of any desired frequency. Ifcoupled with another triggering circuit, monostable and bistableoscillators may also be used. The multivibrator 40 is coupled to thegate of thyristor 20 by a capacitor 42 and resistor 44 and to the gateof thyristor 22 by capacitor 46 and resistor 48 networks, respectively.

Variation of the output frequency of the multivibrator circuit 40 isobtained by the employment of variable resistors 49 and 50 (ganged at51) which are, respectively, positioned in each of the base circuits ofthe transistors. Such variable control of multivibrator output frequencyproduces a consequent controllable output from the pulse output circuit15 which results in output frequency control of power delivered to thetreating load circuit 52.

The output of the transformer thyristor section of the high voltagegenerator is essentially a pulsed wave of variable frequency. Suchoutput is produced by sequentially gating thyristors 20 and 22 by timingpulses applied to the gates thereof by the timing circuit 60.

More particularly, when thyristor 20 is closed, thyristor 22 ismaintained in a blocked or open condition and current from the powersupply will then flow through the inductor 38 and one half of thetransformer. The capacitor 28 is across the whole transformer. Thisseries combination of inductor 38 and capacitor 28 oscillate (at afrequency higher than the gating frequency) to provide a single cycle ofoscillation. The thyristor 20 is self-extinguished during the time thatdiode 34 is conducting (i.e., the negative portion of the cycle). Eachthyristor is independently turned off by this procedure.

When thyristor 22 is closed, the same sequence occurs using the otherhalf of the transformer in a sequential manner. By this action, currentfrom the power source alternately flows through the two sides of thetransformer primary as the thyristors are sequentially fired.

Since the direction of current flow through the two halves of theprimary is opposed, an alternating, variable frequency, pulse waveoutput having an amplitude of about [N /N 2 E wherein N is the number ofwindings on the secondary of the transformer and N is the number ofwindings on each half of the primary, will be created in the secondaryhaving the waveform shown schematically in FIG. 2(a) of the drawings.This voltage is applied to the treater circuit load and produces atreater load current having a pulse waveform as shown schematically inFIG. 2(b) of the drawings.

The waveshape of the voltage output from the secondary of thetransformer is an alternating pulse superimposed on a residual pedestal.This pedestal is due to the charge remaining on the system capacitanceat the end of each pulse. The load current in the treater circuit hasthe waveform of a series of alternatingdirectional, sonic frequency,single oscillation pulses. There is a natural resonant ring due to thetransformer following the useful load current burst. This ring does notcontribute to corona. Comparing the waveforms of FIGS. 2a and 2b inproper time sequence one can see that the current (2b) is a derivativefunction of the voltage (2a).

The solid state high voltage generating system disclosed herein isespecially suited for use in polymer film treating systems. As shownschematically in FIG. 1, the system as a whole consists of the highvoltage generator whose output is connected to the film treating workcell 52 comprising a treater electrode 54 which is normally separatedfrom ground electrode 56 by an air gap 58, the polymeric film 60 and abuffer dielectric 62.

To effectively modify or treat the surface of a polymeric film, thesolid state, variable frequency, high voltage generator must cause arapid sequence of high voltage gaseous discharges to occur in gap 58during passage of a polymeric film therethrough.

In carrying out treating tests in accordance with the present invention,a high slip polyethylene film inches wide, 1.5 mil thick, traveling at50 feet per minute, was exposed to the corona discharge provided by thepulse generator of FIG. 1. It was possible to operate over an extremelywide frequency range. The voltage fed to the corona generator wasmaintained at volts dc and the input current varied with variations infrequency as a criterion of loading. By taking the product of dc voltageand dc current, the power input to the generator is indicative ofloading.

The observed data is presented as the curves of FIG. 3. In the curves,the number associated with each curve indicates the length (in inches)of the electrode employed for treating.

The load is continuously controllable down to essentially zero. Theimpulsive waveshape has resulted in lower harmonic currents and theirassociated resonances. It would be expected that in avoiding anyresonance absorption, circulating currents and the associated internalheating would be reduced and it was noted that certain componentsoperated cooler.

Tests were carried out at desired energy densities and the treatment wassatisfactory for ink adhesion at commercial levels.

The term high voltage gaseous discharge, as used herein, applies to thedischarge phenomenon observed during the treatment of polymer films.Although essentially a suppressed are which possesses aspects of coronaglow and are discharges, the predominant visual indicia is the coronawhich has caused the art to term the phenomenon a corona discharge."

To generate the high voltage discharge in the gap 58, the high voltagegenerator is capable of supplying to a sharp knife-edge electrode atleast 2,000 volts ac. Commercial units with larger radius electrodesrequire from about 5,000 to 50,000 volts or more ac which for a dc powersupply having an output up to about 120 volts dc will require atransformer having at least 20, preferably 70 or more, windings for eachhalf primary winding. It will be appreciated, however, that the numberof sec ondary windings could vary depending on the magnitude of theselected supply voltage. The solid state high voltage generator shouldalso be capable of providing a power output of from about 5 to about 25watts per linear inch of electrode 54 to effectively treat the surfaceof a polymeric film.

Since polymer film treating systems operate at gap film speeds in orderof about 100 to 200 feet per minute or more, the astable timing circuitshould preferably operate at a frequency of about to 5,000 Hz to closelyspace the discharges on the film surface. As used herein, the term Hz,or Hertz, is the currently accepted abbreviation for cycles per second.

While not critical to the operation of a polymer film treating system,gap spacings in the order of about onesixteenth to three-sixteenth inchare most commonly employed and contemplated within the ambit of thisinvention.

In addition to an efficient duty cycle, and the ability to obtainmaximum loading conditions through frequency control over an extremelybroad range of frequency, the solid state high voltage generator of theinvention possesses several characteristics which are deficient in priorgenerators.

Radio frequency interference is essentially nonexistent because thefundamental waveshape is lacking in radio frequency components. Thisavoids the use of ex-,..

pensive shielding devices and allows its use in areas where regulationshave forbidden the use of other generators.

dc Input voltage variation offers a convenience over existing units.Since the timing circuit operates independently of the voltage supply,output voltage is not dependent on the frequency of the timing circuitand any desired output voltage is available at any selected frequency ofoperation of the timing circuit by variation in do input voltage.Therefore, for any selected dc input voltage, the output voltage of thegenerator will be constant and independent of frequency variation.

What is claimed is:

1. A process for the surface treatment of a plastic body with analternating voltage of high intensity accompanied by corona discharge,wherein the treatment zone constitutes a capacitive load, comprisingexposing said surface to an alternating-directional sequence of pulsewaveform electrical voltage in the sonic frequency range andaccompanying corona discharge, whereby the rapid change of electricalvoltage is maximized and the application of electric stress on allcircuit components is minimized.

2. The process in accordance with claim 1, wherein said sequence ofpulse waveform electrical voltage has a frequency in the range 20-5 ,000Hz.

1. A process for the surface treatment of a plastic body with analternating voltage of high intensity accompanied by corona discharge,wherein the treatment zone constitutes a capacitive load, comprisingexposing said surface to an alternatingdirectional sequence of pulsewaveform electrical voltage in the sonic frequency range andaccompanying corona discharge, whereby the rapid change of electricalvoltage is maximized and the application of electric stress on allcircuit components is minimized.
 2. The process in accordance with claim1, wherein said sequence of pulse waveform electrical voltage has afrequency in the range 20-5,000 Hz.